Czochralski crystallization of gallium arsenide using a boron oxide sealed device

ABSTRACT

Method of producing according to Czochralski, monocrystals of semiconductive compounds easily decomposable at their melting point includes surrounding a crucible containing a melt of such semiconductive compound with a fluid medium extending to a level below the edge of the crucible, immersing a bell which is axially rotatable and vertically displaceable and which has a crystal seed axially suspended therewithin, into fluid medium so as to close off a volume above the surface of the molten compound, lowering the bell further into the fluid medium until the crystal seed is immersed in the melt, and thereafter raising the bell so as to pull the seed crystal and a monocrystalline rod of the semi-conductive compound out of the melt; and device for carrying out the method.

United States Patent [191 Grabmaier [4 1 Feb. 13, 1973 [75] Inventor: Josef Grabmaier, Unterhaching,

Germany [73]- Assignee: Siemens Aktiengesellschaft, Berlin and Munich, Germany 22 Filed: March 13, 1 970 21 Appl. No.: 19,289

[30] Foreign Application Priority Data March 18, 1969 Germany P 19 13 682.4

[52] US. Cl. ..23/30l SP, 23/273 SP [51] Int. Cl .B0lj 17/18, COlg 15/00 [58] Field of Search ..23/30l SP, 273 SP [56] References Cited UNITED STATES PATENTS 2,892,739 6/1959 Rusler ..23/30l 3,078,151 2/1963 Kappelmeyer ..23/273 3,088,853 5/1963 Harper ..23/273 3,198,606 8/1965 Lyons ..23/273 3,235,339 2/1966 Brunet et a1 ..23/30l OTHER PUBLICATIONS Richards, et al., Journal of Scientific Instruments, Vol. 34, July 1957, pp. 289-90 Primary Examiner-Norman Yudkoff Assistant Examiner-11. T. Foster Attorney-Curt M. Avery, Arthur E. Wilfond, Herbert L. Lerner and Daniel J. Tick [57 ABSTRACT Method of producing according to Czochralski, monocrystals of semiconductive compounds easily decomposable at their melting point includes surrounding a crucible containing a melt of such semiconductive compound with a fluid medium extending to a level below the edge of the crucible, immersing a bell which is axially rotatable and vertically displaceable and which has a crystal seed axially suspended therewithin, into fluid medium so as to close off a volume above the surface of the molten compound, lowering the bell further into the fluid medium until the crystal seed is immersed in the melt, and thereafter raising the bell so as to pull the seed crystal and a monocrystalline rod of the semi-conductive compound out of the melt; and device for carrying out the method.

2 Claims, 1 Drawing Figure CZOCHRALSKI CRYSTALLIZATION F GALLIUM ARSENIDE USING A BORON OXIDE SEALED DEVICE My invention relates to method of producing monocrystals of semiconductive compounds readily decomposable at the melting point thereof, such as gallium arsenide especially, wherein the monocrystals are pulled, generally in accordance with the Czochralski method, from melts of the compounds with the aid of seed crystals.

Various ways of producing monocrystals of semiconductive compounds that are readily decomposable at the melting point thereof, especially those containing arsenic as a component, have been tried with greater or lesser success.

A method is known, for example, based on the idea that a given equilibrium vapor pressure of the melting compound in the crucible is maintained by providing a base member of the readily fugitive component, such as arsenic, for example, during the process of hardening in the crucible.

Another known method calls for regulating the vapor pressure by balancing the components instead of by providing a base member.

A further possibility for producing semiconductive compounds readily decomposable at the melting point thereof by employing lower than stoichiometric ratios of the components, i.e. a base member is employed as in the first known method described hereinabove and, in fact, in a way that the melt is not entirely stoichiometrically adjusted but rather, for example, in the production of gallium arsenide compounds, the gallium is present with an excess of several percent.

The same problems that are encountered in the production of the semiconductive compounds also occur in the crystal-pulling thereof. The known methods are suitably modified therefor. The starting point of the monocrystal-pulling for the semiconductive compounds, especially for gallium arsenide, is the Czochralski method. A difficulty encountered therein is to maintain at a high temperature a space completely sealed on all sides, in the interior of which the seed is to be displaced. If no special precautions are taken, the readily fugitive component of the compound, for example arsenic of gallium arsenide, vaporizes out of the melt and deposits on cold locations of the crucible. The vaporization can be avoided by melting the compound in a closed vessel, and all surfaces which define the vaporization chamber in the closed vessel are held at a temperature which is higher than the condensation or sublimation temperature of the readily fugitive component. Pulling of the crystal from the melt is then indeed rendered more difficult because the pulling motion must be transmitted into the closed vessel while all parts of the vessel must be maintained at a relatively high temperature. This problem is solvable by using a magnet system which permits the seed to be displaced. In this method which is quite difficult technically to carry out, it is generally impossible to prevent the formation of a thick layer of the semiconductive compound on the inner surface of the quartz vessel, which greatly obstructs the transparency of the vessel during melting processes that are carried over long periods of time- From the Journal Phys. Chem. 80].," vol. 26, pp. 782 784, a further method is known which proves to be simpler and is known as Encapsulating Technique. The gallium arsenide crystals produced by this known method are free of heavy metals and are therefore better suited for specific semiconductor devices, such as gallium arsenide diodes for very high frequencies (so-called Gunn diodes) for example. The characteristic feature of this known method is that semiconductor material, melted in a quartz crucible and consisting, for example, of an A"B" compound, is fully encapsulated by a thin film of B 0 (boron oxide) and, furthermore, a thicker layer (about 10 mm) of B 0 is superimposed on the encapsulating film. Due to the high viscosity of the B 0 melt and to the excess pressure present in the vicinity of the melt, a protective gas atmosphere (for example, argon at about 1 atmosphere excess pressure) can forestall dissociation and prevent vaporization of the arsenic.

Due to the required encapsulating B 0 impurities, especially oxygen, are dragged into the melt, however, either by direct contact or by washings of the quartz crucible, so that the perfection of the crystal and the purity of the crystal being formed, are considerably impaired.

It is accordingly an object of my invention to provide method and device for producing monocrystals of semiconductive compounds that avoid the foregoing disadvantages of the foregoing known methods of this general type and of the devices for carrying out those known methods. More specifically, it is an object of my invention to provide method and device for producing monocrystals of semiconductive compounds, that are readily decomposable at the melting point thereof, such as gallium arsenide monocrystals, especially, which are of high perfection and purity, through crystallization thereof from a melt having the same composition as that of the crystal.

With the foregoing and other objects in view, I pro vide, according to my invention, method of producing in accordance with the Czochralski principle, a monocrystal of a semiconductive compound relatively readily decomposable at the melting point thereof, which comprises surrounding a crucible containing a melt producing of the semiconductive compound with a fluid medium extending to a level below the edge of the crucible, immersing a bell, which is axially rotatable and vertically displaceable and which has a crystal seed axially suspended therewithin, into the fluid medium so as to close off a volume above the surface of the melt, lowering the bell farther into the fluid medium until the crystal seed is immersed in the melt, and thereafter raising the bell so as to pull the seed crystal and a monocrystalline rod of the semiconductive compound out of the melt.

In accordance with a further feature of my invention, the fluid medium has a low melting point compared to that of the semiconductive compound and has a relatively low vapor pressure. More specifically, in accordance with my invention, substances suitable as the fluid medium, especially if gallium arsenide monocrystals are to be produced, are boron oxide, gallium fused salt baths such as calcium chloride, for example.

In accordance with another feature of the invention, I provide an outer crucible for receiving therein the fluid medium surrounding the crucible containing the melt, and including inductively coupling the outer crucible so that it serves as a susceptance for heating the crucible containing the melt. More specifically according to my invention, the outer crucible is made of iridium, rhodium, platinum or graphite.

According to an added feature of the invention and in order to be able to observe the melt, the bell is formed of quartz. However, other impervious substances that are chemically resistant to molten boron oxide are also suitable for the bell.

According to additional features of my invention, the crystal pulling process is carried out in a protective gas atmosphere, such as in an argon or nitrogen current; but it is also possible, in accordance with the invention, to carry out the process in a closed system under high vacuum.

The method of my invention has none of the great disadvantages of the heretofore known methods of this general type:

1. Contamination of the melt due to the fluid medium or due to the protective gas is impossible since there is not contact thereof with the melt;

2. Escape of arsenic during dissociation of the melt is prevented or is permitted at most, to an extent corresponding to the volume of the bell, because the shielding bell always maintains the same temperature during performance of the method. Complex auxiliary heating devices are thereby dispensed with; and

3. Technically, the method is capable of being carried out relatively simply.

In accordance with another feature of my invention, I add dopant additive to the semiconductive melt.

Moreover, the crystal pulling method of my invention is applicable to all substances that dissociate at the melting point thereof and yield readily fugitive components.

In accordance with the device of my invention, l provide device for carrying out the method comprising a crystal pulling system including a first crucible for receiving molten substance therein, the first crucible being received in a second and outer crucible having a diameter greater than that of the first crucible so that the first crucible is able to be surrounded by a fluid medium received in the second crucible and extending up to a level below the upper edge of the first crucible, a bell which is axially rotatable and vertically displaceable and which has a crystal seed axially suspended therewithin is coaxially disposed over the first crucible, the bell having a maximum diameter greater than the diameter of the first crucible, in-

duction heating means located outside the second crucible for heating the first crucible, and means for forming a closed vacuum or a protective gas atmosphere in the crystal pulling system. Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method and device for producing monocrystals of semiconductive compounds, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The invention, however, together with additional objects and advantages thereof will be best understood from the single FIGURE of the drawing which shows schematically and insectional view the device for carrying out the monocrystal producing method of my invention.

Referring now to the drawing, there is shown a reaction vessel 1 wherein a crystal pulling process is to be carried out, the vessel 1 being a quartz tube which is connected, for example, to a vacuum pump or to a source of protective gas, such as argon or nitrogen. In the quartz tube 1, there is located on a crucible pedestal 2 a crucible 3 of platinum which simultaneously serves as an inductively coupled susceptance which, by means of an induction coil 4 surrounding the quartz tube 1 and connected by leads l4 and 15 to an electrical source (not shown), which heats a melt 6 received in a crucible 5 formed of quartz. The crucible 5 is surrounded during the crystal pulling process with a fluid medium 7 having a low melting point compared to that of the melt 6 proper. If the melt 6 consists of gallium arsenide, the fluid medium most advantageously consists of boron oxide (B 0 which, in addition to a low melting point and a low vapor pressure, has the advantage, that it is not miscible with the gallium arsenide.

A quartz bell 8, which closes off a volume above the melt surface, is immersed, during the crystal pulling process, in the liquid medium 7 consisting of B 0 for example. The bell 8 is rotatable about an axial shaft 9 (as shown by the arrow 10) and is vertically displaceable (as shown by the double-headed arrow 11). A crystal seed 12 is secured to the shaft 9 of the bell 8 within the latter and, as the bell 8 is lowered in the fluid medium 7, the crystal seed 12 is immersed in the gallium arsenide melt 6 and as the bell 8 is raised, a monocrystal rod 13, adhering to and growing from the crystal seed 12, is pulled from the melt 6.

The crystal pulling process is performed according to the well known principles of the Czochralski method either in protective gas atmosphere or in high vacuum. Adjustment of the parameters which are otherwise conventional for crystal pulling, such as pulling speed, rotation and crucible supply are effected in a conventional manner.

I claim:

1. Method of producing, in accordance with the Czochralski principle, a monocrystal of gallium arsenide, which is relatively readily decomposable at the melting point thereof which comprises inserting an inner crucible containing a melt of the gallium arsenide into an outer crucible containing a fluid medium consisting of a melt of boron oxide extending to a level below the upper edge of the inner crucible and above the bottom of the inner crucible, inductively coupling the outer crucible so that it serves as a susceptance for heating the inner crucible containing the gallium arsenide melt, immersing a quartz bell, which is axially rotatable and vertically displaceable and which has a crystal seen axially suspended therewithin, into the boron oxide melt so as to close off a volume above the surface of the gallium arsenide melt, lowering the quartz bell farther into the boron oxide melt until the crystal seed is immersed in the gallium arsenide melt and thereafter raisingthe quartz bell so as to pull the edge of said first crucible, an axially rotatable and vertically displaceable quartz bell having a crystal seed axially suspended therewithin being coaxially disposed over said first crucible, said bell having a maximal diameter greater than the diameter of said first crucible and being sealingly immersible in said boron oxide melt, induction heating means located outside said second crucible and coupled thereto for heating said,

first crucible, and means for forming a protective environment within said crystal pulling system. 

1. Method of producing, in accordance with the Czochralski principle, a monocrystal of gallium arsenide, which is relatively readily decomposable at the melting point thereof which comprises inserting an inner crucible containing a melt of the gallium arsenide into an outer crucible containing a fluid medium consisting of a melt of boron oxide extending to a level below the upper edge of the inner crucible and above the bottom of the inner crucible, inductively coupling the outer crucible so that it serves as a susceptance for heating the inner crucible containing the gallium arsenide melt, immersing a quartz bell, which is axially rotatable and vertically displaceable and which has a crystal seen axially suspended therewithin, into the boron oxide melt so as to close off a volume above the surface of the gallium arsenide melt, lowering the quartz bell farther into the boron oxide melt until the crystal seed is immersed in the gallium arsenide melt and thereafter raising the quartz bell so as to pull the seed crystal and a monocrystalline rod of gallium arsenide out of the gallium arsenide melt. 